Recently technological application of nanocarbon has been actively investigated. The nanocarbon is a carbon substance having a nanoscale fine structure, typified by a carbon nanotube, a carbon nanohorn, and the like. The carbon nanohorn has a tubular structure like a carbon nanotube, formed by a cylindrically rounded graphite sheet and one end of the carbon nanohorn is formed in a circular conic shape. The carbon has a specific character and is expected to be applied to various technical fields. Usually the carbon nanohorn is aggregated in a form so that the circular conic portion is projected like a horn while the tube is located in the center by Van der Waals force between circular conic portions. Hereinafter the aggregation is referred to as “carbon nanohorn assembly.”
It has been reported that the carbon nanohorn assembly is produced by a laser ablation method of irradiating the carbon substance (hereinafter also referred to as “graphite target”) of a raw material with a laser beam in an inert gas atmosphere (Patent Document 1).
However, in the conventional laser ablation method, production efficiency is not sufficiently improved, and there is a problem in large-scale production. Particularly, purities of the carbon nanohorn included in the product are not sufficient, that causes a long time for a purification process. Graphite and amorphous carbon are contained in soot generated by the laser ablation method besides the single-layer carbon nanohorn. In order to industrially utilize the carbon nanohorn, it is necessary that the impurities be removed through the purification process. Usually the purification is performed by an oxygen treatment. However, it takes a long time to perform the purification process. For example, currently the purification of the soot of 10 grams is required one day or more, and it is difficult to efficiently obtain the carbon nanohorn.
Further, fluctuations in quality and property of the obtained carbon nanohorn are relatively large in the conventional method, so that there is still room for improvement from the viewpoint of quality stability.
For example, in the conventional technology, it is difficult to obtain carbon nanohorns with even lengths. Further, it is extremely difficult to controll properties of the carbon nanohorn depending on the horn length.
There is also the problem that structures except for the carbon nanohorn remain in the carbon nanohorn assembly. According to TEM (Transmission Electron Microscope) observation performed by the inventors, it comes obvious that densely aggregated graphite sheets, having no horn structure, exists in the central portion of the carbon nanohorn assembly produced by the conventional method. That is, after oxidation heat treatment is performed at 575° C. to the carbon nanohorn obtained by the conventional laser ablation method, when the TEM observation is performed, it is found that a point which is not oxidized exists in the central portion of the assembly while the oxidation progresses in a surface portion. This suggests that the dense structure of the graphite sheet having no horn structure exists in the central portion of the carbon nanohorn assembly produced by the conventional method.
Patent Document 1: Japanese patent publication laid-open No. 2001-64004